The present invention pertains to an induced polarization logging system such as described in U.S. Pat. No. 4,359,687 by Harold J. Vinegar and Monroe H. Waxman, issued Nov. 16, 1982. In this patent there is described an induced polarization logging tool and method for determining the cation exchange capacity per unit pore volume Q.sub.v, electrolyte conductivity C.sub.w and water saturation S.sub.w of shaly sand formations using in situ measurements. In particular, the patent described a logging tool having an insulated sonde with current and return electrodes together with voltage measurement and reference electrodes and means to determine both the in-phase and quadrature conductivity. The induced polarization logging tool described in U.S. Pat. No. 4,359,687 provides means for determining the value of Q.sub.v, C.sub.w and S.sub.w from in situ measurements thereby greatly improving the evaulation of a formation penetrated by the borehole.
The above mentioned invention has several limitations, however. The presence of the borehole filled with conductive drilling mud requires use of borehole correction curves to obtain a true indication of the actual formation parameters. When the formation resistivity is considerably greater than the mud resistivity, very large borehole corrections are required. The problem is compounded in the case of formations heavily invaded by mud filtrate, where several different array spacings are required for a complete formation evaluation. Another problem occurs in logging thin formations, where the measured induced polarization is only a fraction of the true formation values. The formation thickness must be several times the AM array spacing in a Normal array to obtain a good approximation to the true formation values. The limitations of the above mentioned logging device arise from induced polarization currents channeling through the borehole and invaded zones, rather than into the uninvaded formation adjacent to the tool.
The present patent discloses a method whereby the induced polarization currents are focused into the uninvaded formation. In the present invention, two focusing electrodes are placed on opposite sides of the survey current electrode. The focusing electrodes inject current which is exactly in phase with the current injected at the survey current electrode. The current applied to the pair of focusing electrodes is controlled to provide zero potential difference between two pairs of monitoring electrodes that are positioned between the survey current and the focusing electrodes. The current injected at the focusing electrodes is supplied from the same current source as the survey current electrode and is controlled by voltage-variable resistors which ensure that the focusing currents and reference currents are exactly in phase.
Although it is common practice in the industry to use focused electrode arrays for resistivity logging, none of the prior art would be suitable for an induced polarization logging tool such as described in U.S. Pat. No. 4,359,687. This is because all prior devices use focusing electrodes which are transformer or capacitor coupled to the current source. Either transformer or capacitor coupling will create phase shifts between the focusing currents and the survey current that will distort the measurement of the formation phase shift, which is extremely small (.about.1 milliradian). This is not a problem in resistivity logging where the phase shift is not measured. Since formation phase shift is the primary quantity measured in the induced polarization logging tool of U.S. Pat. No. 4,359,687, this would prevent obtaining accurate values for Q.sub.v, C.sub.w and S.sub.w using insitu measurements.